This is a very interesting article about flight speed in birds and what affects it. One of the things that does is the size of the flock! The larger the flock, the faster they fly - isn't that something?!

Amazing, I think that the reason for the change in speed is simple. If you have more birds in the flock, they have more birds in front to break the air molecules and make the air less dense decrease resistance for the birds behind so is easier for birds to push their bodies through the air and increase speed.

Hi Pajarita. It's working for the ones in front as well, because the birds who are behind,; while propelling their bodies forward, push some air forward and create tail wind for the birds in front. So is win win situation. That is why dolphins swim in front of moving boat because moving boat push some water forward and create wave so they can surf and save some energy . And I also know that from personal experience, I am Paragliding pilot and whenever we fly in formation we extend our glide.